Raman scattered light is considered a chemical signature of chemical and biological molecules since all molecules with unique chemical compositions have unique Raman scattering spectra. Raman scattering spectroscopy is thus a powerful technique to detect chemical and biological molecules without labeling and has potential applications in high-sensitivity detections of explosives, pathogens, and contaminants in the field. Unfortunately, Raman scattering is an extremely inefficient process due to its weak sensitivity as compared to other scattering mechanisms (roughly only 1 in 108 photons ends up in Raman scattering) [Jarvis and Goodacre, 2004 Anal. Chem. 76 40] and also due to typical lower scattering cross-sections of Raman process (˜10−30 cm2), which is around 15 orders of magnitude lower than fluorescence emission [Vo-Dinh et al., 2002 J. Raman Spectrosc. 33 511]. In order to get detectable Raman scattering, it is necessary to use an array of filtering techniques or to enhance the Raman scattering process. The latter of the two can be achieved using surface enhanced Raman scattering, which is also known as surface enhanced Raman scattering spectroscopy or surface enhanced Raman spectroscopy. For the sake of clarity, throughout the present disclosure, the term “SERS” intends to indicate surface enhanced Raman scattering.
Back in the 1970's, many scientists came to discover a new phenomenon in Raman scattering, which is now widely referred to as SERS. When chemical and bio-molecules are adsorbed on a roughened noble metal surface, the Raman scattering light can be enormously amplified and the sensitivity of the Raman spectroscopy enhanced by several orders of magnitude. One of the many approaches that has been tried includes microscale or nanoscale “roughening,” such as in the form of electrochemical texturing of a surface before metal sputtering [Murray et al., 1981 Phys. Rev. Lett. 46 57]. In general, reported signal enhancements have been significant. However, enhancement is observed only at so-called “hot spots,” regions where the Raman signal is higher, while neighboring regions might exhibit little or even no significant signal enhancement.
For the sake of clarity, throughout the present disclosure, the term “hot spot”, “hot spots”, or “SERS hot spots” intends to indicate regions where the Raman signal is higher, or enhanced. The low concentration of hot spots within a sample is exacerbated further by an inconsistency of performance between different but essentially identical substrates, or sample-to-sample uniformity [Netti et al., 2005 Raman Technology for Today's Spectroscopists]. Overall, these two aspects together effectively have prevented SERS from being widely recognized as a quantifiable spectroscopic technique [Etchegoin and Le Ru, 2008 Phys. Chem. Chem. Phys. 10 6079-6089].